From LoveToKnow 1911

'KASAI, or Cassai, a river of Africa, the chief southern affluent of the Congo. It enters the main stream in
3° so' S., 16° 16' E. after a course of over Boo m. from its source
in the highlands which form the south-western edge of the Congo
basin - separating the Congo and Zambezi systems. The Kasai and its many
tributaries cover a very large part of the Congo basin. The Kasai
rises in about 12° S., 19° E. and flows first in a north-easterly
direction. About so°35 S., 22° s5'
E. it makes a rectangular bend northward and then takes a north-westerly direction. Five
rivers - the Luembo, Chiumbo, Luijimo or Luashimo, Chikapa and
Lovua or Lowo - rise west of the Kasai and run in parallel courses
for a considerable distance, falling successively into the parent
stream (between 7° and 6° S.) as it bends westward in its northern
course. The Luembo and Chiumbo join and enter the Kasai as one
river. A number of rapids occur in these streams. A few miles below
the confluence of the Lowo, the last of the five rivers named to
join the Kasai, the main stream is interrupted by the Wissmann
Falls which, though not very high, bar further navigation from the north. Below this
point the river receives several right-hand (eastern) tributaries.
These also have their source in the Zambezi-Congo watershed, rising just
north of 12° S., flowing north in parallel lines, and in their
lower course bending west to join the Kasai. The chief of these
affluents are the Lulua and the Sankuru, the Lulua running between
the Kasai and the Sankuru. The Sankuru makes a bold curve westward on reaching 4° S.,
following that parallel of latitude a considerable distance. Its waters
are of a bright yellow colour. After the junction of the two rivers
(in 4° 17' S., 20° 15' E.), the united stream of the Kasai flows
N.W. to the Congo. From the south it is joined by the Loange and
the Kwango. The Kwango is a large river rising a little north of
12° S., and west of the source of the Kasai. Without any marked
bends it flows north - is joined from the east by the Juma, Wamba
and other streams - and has a course of 600 m. before joining the
Kasai in 3° S., 18' E. The lower reaches of the Kwango are
navigable; the upper course is interrupted by rapids. On the north
(in 3° 8' S., 17° E.) the lower Kasai is joined by the Lukenye or
Ikatta. This river, the most northerly affluent of the Kasai, rises
between 24° and 2 5° E., and about 3° S. in swampy land through
which the Lomami (another Congo affluent) flows northward. The
Lukenye has an east to west direction flowing across a level
country once occupied by a lake, of which Lake Leopold
II., connected with the lower course of the Lukenye, is the
scanty remnant. Below the lake the Lukenye is known as the Mfini.
Near its mouth the Kasai, in its lower course generally a broad
stream strewn with islands, is narrowed to about half a mile on
passing through a gap in the inner
line of the West African highlands, by the cutting of which the old
lake of the Kasai basin must have been drained. The Kasai enters
the Congo with a minimum depth of 25 feet and a breadth of about
700 yards, at a height of 942 ft. above the sea. The confluence is
known as the Kwa mouth, Kwa being an alternative name for the lower
Kasai. The volume of water entering the Congo averages 3 21,000
cub. ft. per second: far the largest amount discharged by any of
the Congo affluents. In floodtime the current flows at the rate of
5 or 6 m. an hour. The Kasai and its tributaries are navigable for
over 1500 m. by steamer.

The Kwango affluent of the Kasai was the first of the large
affluents of the Congo known to Europeans. It was reached by the
Portuguese from their settlements on the west coast in the 16th
century. Of its lower course they were ignorant. Portuguese
travellers in the 18th century are believed to have reached the
upper Kasai, but the first accurate knowledge of the river basin
was obtained by David Livingstone, who reached the
upper Kasai from the east and explored in part the upper Kwango
(1854-1855). V. L. Cameron and Paul Pogge crossed the upper Kasai
in the early "seventies." The Kwa mouth was seen by H. M. Stanley in his journey down the
Congo in 1877, and he rightly regarded it as the outlet of the
Kwango, though not surmising it was also the outlet of the Kasai.
In 1882 Stanley ascended the river to the KwangoKasai confluence
and thence proceeding up the Mfini discovered Lake Leopold II. In 1884
George Grenfell journeyed up the river beyond the Kwango
confluence. The systematic exploration of the main stream and its
chief tributaries was, however, mainly the work of Hermann von
Wissmann, Ludwig Wolf, Paul Pogge
and other Germans during 1880-1887. (See Wissmann's books,
especially Im Innern Afrikas,Leipzig, 1888.) On his third journey, 1886,
Wissmann was accompanied by Grenfell. Major von Mechow, an
Austrian, explored the middle Kwango in 1880, and its lower course
was subsequently surveyed by Grenfell and Holman Bentley, a Baptist
missionary. In1899-1900a Belgian expedition under Captain C.
Lemaire traced the Congo-Zambezi watershed, obtaining valuable
information concerning the upper courses of the southern Kasai
tributaries. The upper Kasai basin and its peoples were further
investigated by a Hungarian traveller, E. Torday, in 1908-1909.
(See Torday's paper in Geog. Jour., 1910; also CONGO and
the authorities there cited.)